The present invention relates to single phase brushless DC motors, i.e. a motor with a single coil. Single-phase motors are typically used in low cost motor applications, such as fan cooling applications.
Methods and circuits for driving single coil motors are known in the art, for example from US2011181214(A1) and U.S. Pat. No. 7,639,064(B2).
FIG. 1 shows a prior art system 100 comprising a single coil BLDC motor having a stator with a coil, and a rotor with permanent magnets (for example 2 north poles and two south poles), and a Hall sensor 102 for sensing whether a north pole or a south pole is passing the position of the Hall sensor. A controller 101, for example comprising an analog or a digital state machine, can read the Hall sensor signal and provide control signals C11, C12, C21 and C22 to drive the low-side transistors T11, T21 and the high-side transistors T12, T22 of the dual-H bridge (see FIG. 1a) in manners known per se in the art.
In some applications, e.g. fan cooling applications, the motor is designed to run always at maximum speed (when power is provided). Even though in these applications the speed is not configurable, the controller needs to make sure that the correct transistors are switched on and off, depending on the actual rotor position. The switching of the transistors is known as “commutation”, which is a well known principle used to drive brushless motors.
FIG. 2(a) shows an exemplary (ideal) current waveform 201 with abrupt changes, obtainable by a so called “hard switching” technique. When using such a control scheme, the current in the coil changes very fast from a maximum positive value to a maximum negative value after each 180° of rotation (known as “half phase”), and remains constant between these “commutation points”.
FIG. 2(b) shows another exemplary waveform 202 obtained by a so called “soft switching” technique, which can be achieved for example by driving the transistors in their linear region, or by driving at least some of the transistors with a PWM-signal. Many variations are possible. For example, the slope of the rising edge (typically after the moment commutation) and the slope of the falling edge (typically before the moment of commutation) need not be the same, and there may be a delay between the end of the falling edge and the start of the rising edge, etc. A portion of the waveform 202 of FIG. 2(b) is also shown in FIG. 2(c) in enlarged view.
FIG. 2(c) also shows a portion of a third exemplary waveform 203, where “the tail” of the waveform changes abruptly, then remains zero for ⅛ of 180°, and after the commutation point ramps-up linearly to reach its maximum amplitude at 1/16 of 180°. In all of these cases, the waveforms need to be synchronized to the actual rotor position in one way or another, which is a challenge to achieve, especially for low-cost fan drivers.